The present invention relates generally to semiconductor manufacturing, and more particularly, to a method of manufacturing a semiconductor device by crystallizing an amorphous silicon film through the use of a heat retaining layer.
The demand for smaller electronic products with higher resolution displays spurs continued research and development in the area of liquid crystal displays (LCDs). An important component of an LCD is a thin film transistor (“TFT”). TFTs typically are fabricated on a transparent substrate such as glass or plastic, and employ a thin film silicon semiconductor.
Thin film silicon semiconductors typically include amorphous silicon (a-Si) semiconductors and polycrystalline silicon (p-Si) semiconductors. Amorphous silicon semiconductors are most commonly used because they can be fabricated relatively inexpensively through mass production. P—Si semiconductors have superior physical properties, such as electrical conductivity, when compared to a-Si semiconductors. P—Si semiconductors, however, are more difficult to produce than a-Si semiconductors. Due to the superior properties of p-Si semiconductors, much research has been focused on improving the process of fabricating p-Si semiconductors.
A method for manufacturing a p-Si TFT LCD includes a process of crystallization. The process includes forming an a-Si thin film on a glass substrate, and forming the thin film into a p-Si film. Examples of conventional methods of crystallization include a solid phase crystallization (“SPC”) method and an excimer laser annealing (“ELA”) method. These methods use low thermal budgets, i.e., high temperatures and short annealing times for the ELA method, or relatively low temperatures with longer annealing times for the SPC method. The low thermal budget process is important because glass substrates typically used in LCDs cannot withstand high annealing temperatures and/or long annealing times without deforming or breaking. For example, glass typically breaks or deforms at temperatures of approximately 600° C. (the highest temperatures typically allowed without deforming the glass substrate).
The SPC method includes annealing the a-Si film formed on the glass substrate at a temperature of approximately 400° C. to 800° C. so as to obtain a p-Si film. Since the SPC method may involve the use of a high-temperature process, it is necessary to use an expensive quartz glass in the glass substrate, typically is cost prohibitive for use in LCD devices.
The ELA method includes irradiating the a-Si film with a short-pulse excimer laser having a pulse width of about 20 to 200 ns so as to obtain the p-Si film. The ELA method is currently the most commonly used method in polycrystalline silicon TFT fabrication. The grain size of p-Si thin film can reach 300 to 600 nm, and the carrier mobility of p-Si TFTs can reach 200 cm2/V-s. However, this value is yet not sufficient for future demand of high-performance flat panel displays. Furthermore, unstable laser energy output of ELA narrows down the process window, generally to a level of several tens of mJ/cm2. Therefore, frequently repeated laser irradiation is necessary to re-melt imperfect fine grains caused by the irregular laser energy fluctuation, which renders the ELA method cost-ineffective and in turn less competitive.
In the p-Si TFT LCD, either manufactured by the SPC method or the ELA method, there has been a stronger demand for further enlargement of the crystal grain size of the p-Si film. In the conventional methods, the grain size is approximately 300 to 600 nm. To enhance capability of the p-Si TFT LCD, it may be necessary to enlarge the grain size to several micrometers or more. Reasons for the enlargement of the crystal grain size are described below. There is a numeric value of mobility as a factor which influences the capability of a semiconductor device. The mobility represents an electron drift velocity. The drift velocity drops when the grain size is small, and there are many crystal grain fields in a path of the electron. When the drift velocity decreases, the enhanced capability of the semiconductor device cannot be expected.
A technique called super lateral growth (“SLG”) has been known to form crystal grains of larger grain size, as compared to the crystal grain size via conventional excimer laser crystallization. Although a relatively large crystal grain can be obtained, the energy intensity area of a laser beam, where the SLG is obtained, is much higher than the intensity that is used in ordinary excimer laser crystallization. Also, the range of the energy intensity area is extremely narrow. From the viewpoint of the position control of the crystal grain, it may be impossible to control the position where a large crystal grain is obtained. Consequently, the size of the crystal grains is not even and the roughness of the crystal surface is extremely large.
A Sequential Lateral Solidification (“SLS”) method therefore has been developed to control artificially the SLG to be taken place at a desired position. According to the SLS method, an excimer laser beam of pulse oscillation is irradiated to a material via a slit-like mask. The crystallization is carried out while the relative position between the material and the laser beam is displaced, at every shot, by a distance which is roughly equivalent to the length of the crystal formed via the SLG. However, the SLS method may suffer from an insufficient throughput due to a relatively small feed pitch, i.e., the relative shift distance, of the laser beam between the beam spot on the material surface and the material substrate.
It is therefore desirable to have a method of semiconductor thin film crystallization that can achieve higher throughput and greater, uniform grain size in a cost efficient manner without compromising desired electrical properties.